Hello I am new to Apex / Trigger and would like to use it to update one field from another on a custom object. I know this can be done via workflow but would like to try this with  a trigger so I can start to understand more about Apex. I am in the mids of reading about apex etc but would be grateful if someone could give me a few examples.

What I am trying to achieve:

Object name = Test object

Field 1 = A

Field 2 = B

All I want to do is make "field A" update with "field B" value after update.

Thank you